The fundamental operation of pulsed radars for measuring the distance and velocity of objects had already been described in 1938 by Col. William Blair of the U.S. Signal Corps. When a microwave carrier undergoes pulse modulation, a signal of a defined pulse duration Tp is periodically transmitted at the pulse repetition frequency PRF. The signal reflected off of an object is attenuated in the receiver to the baseband range. By analyzing the baseband signal, the signal propagation time T and possibly the signal Doppler shift fD are determined. From the propagation time T, the object-sensor slant distance R is derived, ultimately based on the speed of light c, from the relationship R=c·T/2, and the object velocity v is determined, with the carrier frequency being fC, from the relationship v=c/2·fD/fC.
Conventional pulsed radar systems use the following operating modes:
LPRF (Low Pulse Repetition Frequency):
In this case, pulsed radars have such a low pulse repetition frequency PRF, that a unique measurement up to the greatest desired object distance is always possible. However, if velocities occur that can result in fD being greater than PRF/2, the velocity determination is no longer unique.
HPRF (High Pulse Repetition Frequency):
Here, operation takes place at such a high pulse repetition frequency that the velocity determination in the entire relative velocity range is always unique. The distance measurement is only unique when all objects in the detecting range exclusively have smaller distances than c/(2 PRF) to the sensors.
PRF Staggering (Staggered PRF):
To avoid so-called blind speeds which occur at constant pulse repetition frequency, or to flatten the line spectrum of the transmitted signal that exists given a constant pulse repetition frequency, e.g., for improved interference suppression, pulse pause intervals of variable length (variable interpulse period VIP) are also used.
Coherent Mixing:
To attenuate the received signal to the baseband range, it is customary for the receiver to mix the received signal with a copy of the transmitted signal. Given a spatial proximity of the transmitter and receiver, the copy can be possibly derived from the same oscillator as the transmitted signal or from a second oscillator of the receiver's own. Depending on whether a stochastic relationship exists from pulse to pulse, among the phases of the received signal and its copy, one speaks of incoherent or coherent mixing. The coherent mixing affords a precise Doppler or velocity determination. However, to achieve the desired coherence, considerable outlay must be expended to synchronize the phases (e.g., use of lock-pulse methods or digital detectors of the transmission phase). Incoherent methods are usually called for when no velocity measurement or only an imprecise velocity measurement is required.
Monostatic, Bistatic:
If the transmitting and receiving antennas are “distinctly” spatially distant from one another, and if the transmitted signal and its copy are derived from different oscillators for mixing purposes, one usually speaks of bistatic radar systems, in contrast to monostatic radar systems.
Pulse Compressions:
For a pulsed radar to achieve a minimal coverage range, a minimum of total energy is required which must be reflected off of an object and integrated by the receiver. Given a predefined pulse repetition frequency, limited peak power output of the transmitter, and limited permissible integration time, the energy can only be still increased by prolonging the pulse duration. On the other hand, the correlation duration (width of the autocorrelation function) of a pulse determines the attainable resolution of a pulsed radar. By using internal pulse modulation/coding, also referred to as pulse compression methods, the correlation duration (the resolution) and pulse duration (energy and average power output and, thus, instrumented coverage range) can be theoretically defined independently of one another. Customary compression methods are linear or non-linear frequency modulation, as well as biphase or multiphase modulation.
It is known that varying combinations and hybrid forms of the above mentioned methods are used.
Fields of Application of Pulsed Radars:
Monostatic Pulsed Radars:
In military applications and in civilian air-traffic control, e.g., monostatic pulsed radars having substantial transmitting power and antenna directivity (beam focusing) are often used for measuring great distances and, to some extent, high velocities. Frequently, a range and azimuth scan is carried out, as well as a relatively complex Doppler processing (MTI (moving target indication), MTD (moving target detection) process), as well as, typically, pulse coding/pulse compression, e.g., chirp (dynamic wavelength change) and modulation of the pulse repetition frequency (VIP (variable interpulse period), staggered PRF (pulse repetition frequency)).
Bistatic Pulsed Radars:
Bistatic pulsed radars are found in military applications, in astronomy and in meteorology, where the object distances are large and are accompanied by great transmitter and receiver distances (for example, baselines in the range of hundreds of kilometers). High demands are typically placed on the components of such bistatic radars, particularly due to the requisite time synchronization of the sensors (pulse synchronization for distance measurement, phase synchronization for velocity measurement (Doppler)) over large spatial distances. Also regarded as difficult are the required synchronized alignment of the viewing directions and, in some instances, allowance for platform movements.
Low-cost Pulsed Radars:
Microwave pulsed radars are increasingly being used in applications where objects are detected at small distances, using low transmitting power and a wide visual range, and where, additionally, low costs are required, such as for door openers, room surveillance, detection of motor-vehicle surrounding fields. Often used in this context are monostatic LPRF (low pulse repetition frequency) methods, incoherent mixing, no pulse compression, or possibly pulse compression including biphase modulation. In contrast to military radar systems or air-traffic control radars, for the low-cost pulsed radars, high-quality components are rarely used. Rather, oscillators having low frequency stability, mixers and LNAs (low-noise amplifiers) having low bandwidth and high noise factor are used, for example.